Generally, a gas turbine is one kind of turbo machines in which fuel is burnt by using high pressure compressed air and the high temperature and high pressure gas generated during the burning process is used to produce rotary power.
The gas turbine largely includes a compressor adapted to suck external air to provide a high pressure stream of air through compression of the sucked external air, a combustor adapted to mix fuel and the high pressure air compressed through the compressor and to burn the mixed fuel and air, and a turbine adapted to generate a rotary force for producing energy from the flow of high temperature and high pressure combustion gas discharged from the combustor.
Further, the leakage of combustion gas from the turbine to the outside, not via blades, gives substantially bad influences on the whole efficiency of an engine, and accordingly, it is important to solve the above-mentioned problem.
FIG. 1 shows a conventional gas turbine and the leakage deficiencies thereof. Referring to FIG. 1, a turbine 71 includes blades 75 rotating at a high speed with respect to a rotary shaft by means of a flow of combustion gas, and the leakage of the combustion gas is generated on a clearance between the free end portion of the blade 75 and a casing 77. The clearance is called a tip clearance G. The casing 77 includes an outer casing 77a having an inwardly bent groove formed thereon and an inner ring segment 77b having as shape coupled with the inwardly bent groove of the outer casing 77a. 
On the other hand, the minimization of the tip clearance G is important to increase the efficiency of the gas turbine. By the way, if tolerances in the coupling between the outer casing 77a and the inner ring segment 77b are accumulated, it is hard to control the tip clearance G. If the tolerance occurs, that is, the outer casing 77a or the inner ring segment 77b itself should be machined again. In this case, the machining cost and time undesirably causes the loss in the whole process.
So as to control and minimize the tip clearance G, in conventional practices, the casing 77 itself is precisely machined. However, tolerance stacking occurs on the assembled parts, and further, it is impossible to additionally control the tip clearance G during the assembling process.